In Chapter 4, the Jelly Bean Counter can be found at the Spring Festival. Talk to him and guess "800", then read the woman's emotions. Next, talk to the guy again and guess "700". Read the woman's thoughts one more time to learn the answer and speak to the guy again and tell him "731".
Timestamps:
0:00 - Memory: Pressed Rose (Scene: Party Time)
0:35 - Side Quest: Jelly Bean Counter (Scene: The Spring Festival) [Exactly 731 Achievement / Trophy]3:00 - Memory: Drum Kit (Scene: The Spring Festival)
3:30 - Memory: Microphone (Scene: The Spring Festival)
4:20 - Memory: CIT Sticker (Scene: The Spring Festival)
5:08 - Memory: Polaroid (Scene: The Spring Festival)